Thin film el device and method of manufacturing the same

ABSTRACT

The thin film EL device includes a luminescent layer made of column polycrystals formed by independently evaporating luminescent host material and an activator and then combining evaporated substances on a substrate. 
     Electrons in the luminescent layer are accelerated by electric field applied from outside efficiently collide against the activator without being intercepted by interfaces between crystalline particles. The thin film EL device can be used for display, illumination, writing, reading out and erasure of signals of photo- recording medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a thin film electroluminescent (EL) elementand more particularly an improved high brightnessllow voltage drive thinfilm EL device, and a method of manufacturing the same.

2. Description of the Prior Art

A prior art dispersion type EL device using a powder of zinc sulfide(ZnS) type fluorescent substance could not be used as a light source ofillumination because of its low brightness. In recent years, a thin filmtype EL device utilizing a thin film of fluorescent substance has becomenoted because of the high brightness.

The thin film EL device is now widely used for vehicles, displayapparatus of a computer terminal element or the like and a source ofillumination because the thin film EL device comprises a light emittinglayer made of a transparent film which prevents dispersion of lightincident from outside or emitted by an internal luminescent layerthereby decreasing halation and blur and providing clear display of highcontrast

For example, a thin film EL device utilizing Mn as a activator in ZnShas a dual dielectric structure wherein a transparent electrode made ofa tin oxide layer (SnO₂), a first insulating layer, a crystalline filmutilizing ZnS as a host material and Mn acting as an activator(anactivator), that is an ZnS:Mn luminescent layer, a second insulatinglayer and a back electrode made of aluminum or the like are sequentiallylaminated on a transparent substrate.

Light is emitted as follows. When voltage is impressed across thetransparent electrode and the back electrode, electrons that have beentrapped at an interface level by electric field induced in theluminescent layer are released and imparted with energy sufficient toaccelerate the electrons so that these electrons collide against theorbit electrons of Mn (luminescent center) so as to excite the orbitelectrons. Light is emitted when the excited activator returns to thebase state.

As disclosed in Japancse Published Patent Specification Nos. 10358/1978and 8080/1979, the electron beam vapor deposition method has been usedfor forming such luminescent layer as ZnS:Mn of the thin film EL device.

Such thin film EL device has been formed in a vacuum chamber 1 byirradiating a pellet 2 formed by sintering a mixture of ZnS and 0.1-1at. % of Mn with an electron beam 4 emitted by an electron gun 3 asshown in FIG. 10, so as to heat and vaporize the pellet and cause thevaporized pellet to deposit on a substrate 5.

With this method, however, since the vapor pressure of the host materialcomprising the luminescent layer, the vapor pressure of an elementcomprising the host material, and the vapor pressure of the activator(for example PZnS, PZn, PS, PMn) differ greatly (PZnS<PMn<PZn<PS) thereare such problems that the host material of the laminated luminescentlayer deviates from stoichiometric composition, thus degradingcrystalline structure and making nonuniform the distribution of theactivator due to nonuniform evaporation and reevaporation of elementsonce deposited on the substrate. In the foregoing description PZnS, PMn,PZn and PS respectively represent vapor pressures of ZnS, Mn, Zn and S.

Consequently, as shown in FIG. 11, the luminescent layer formed by theelectron beam vapor deposition method has a particulate polycrystallinestructure or at the initial stage of growth many small crystal particlesare formed, that is a structure in which so called dead layers present.

In a thin film EL device utilizing such luminescent layer, electrons Ein the luminescent layer accelerated by electric field applied fromoutside are collide against the activator Im with the result that theelectrons will be dispersed at the interfaces B of the crystal particlesbefore the electrons contribute to luminescence. Thus, the electricfield applied from outside does not efficiently contribute toluminescence.

Efficient deriving out of light emitted by the luminescent layerisiimportant for increasing the luminescent efficiency. For efficientlyderiving out the light from the luminescent layer, a method ofcontrolling the refractive index and the film thickness of the firstinsulating layer has been proposed as, for example, in JapanesePublished Patent Specification No. 55635 of 1983.

The equivalent circuit of such thin film EL device can be represented bythree serially connected capacitors (see FIG. 4) constituted by a firstinsulating layer 21, a luminescent layer 24 and a second insulatinglayer 25. When the specific dielectric constants εr1 and εr2 of thefirst and secon dielectric layers are sufficiently larger than thespecific dielectric constant εl of the luminescent layer that isεr1,εr2>>εl, thin capacitances Cr1, Cr2, and Cl are expressed by arelation Cr1,Cr2>>Cl so that almost all portions of the voltageimpressed across the element from outside will be applied across theluminescent layer with the result that it is impossible to obtain a highbrightness with a low driving voltage.

For decreasing the driving voltage, it is advertageous to construct thefirst insulating layer with a material having a large dielectricconstant. However, where a material having a large dielectric constantis used, the reflection at the interface between the first insulatinglayer and the transparent electrode becomes large, thus failing toefficiently deriving out light from the luminescent layer.

For this reason, in order to obtain a practical brightness of about 20ft-L, it is necessary to apply a high voltage of the order of 200 V tothe thin film EL device.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a novel thinfilm EL device capable of generating a high brightness and can be drivenwith low voltage, and a method of manufacturing such thin film ELdevice.

According to one aspect of this invention, there is provided a thin filmEL device comprising a transparent electrode, a insulating layer, aluminescent layer and a back electrode, the luminescent layer beingconstituted by column polycrystals.

According to another aspect of this invention, there is provided amethod of manufacturing a thin film EL device including a lamination ofa substrate, a transparent electrode, a insulating layer, a luminescentlayer, and a back electrode comprising the steps of evaporating hostmaterial of the luminescent layer or an element of the host material,and an activator of the luminescent layer from a plurality of discreteevaporating sources, and depositing evaporated host material, or elementthereof and evaporated activator or an element thereof so as to becombined on the substrate.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawings:

FIG. 1 shows the crystalline structure of the luminescent layer of athin film EL device embodying the invention;

FIG. 2 is a diagram showing the principle of the method of forming theluminescent layer according to this invention;

FIG. 3 is a side view showing one embodiment of the thin film EL deviceaccording to this invention;

FIG. 4 shows an equivalent circuit of the element shown in FIG. 3;

FIGS. 5a through 5e are side views showing successive steps ofmanufacturing the element shown in FIG. 3;

FIG. 6a and 6b are graphs showing the result of X-ray diffraction of theluminescent layer of the film EL element of this invention and of theprior art element;

FIG. 7 is a graph for comparing the voltage--brightness characteristicsof the thin film EL device of this invention and of the prior art;

FIG. 8 is a side view showing a modified embodiment of this invention;

FIG. 9 is a graph for comparing the brightness voltage characteristicsof the thin film EL device shown in FIG. 10 and a prior art thin film ELdevice;

FIG. 10 shows the principle of the method of manufacturing theluminescent layer of the prior art thin film EL device; and

FIG. 11 shows the crystalline structure of the prior art luminescentlayer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Briefly stated, according to this invention a column polycrystallinefilm is used for luminescent layer of a thin film EL device. Further,the host material of the luminescent layer or the elements constitutingthe base material and an activator are evaporated from independentsources and the evaporated substances are then combined on thesubstrate.

Thus as shown in FIG. 1, where column polycrystals are used as theluminescent layer, the electron E in the luminescent layer areaccelerated by the electric field applied from outside and collideagainst the actuator Im, thereby enabling efficient luminescence.

Referring to FIG. 2, to form the luminescent layer, constitutingelements of host material and activator, for example, Mn, S and Zn areput in separate crucibles 12, 13 and 14 in a vacuum chamber 11 which ismaintained at a vacuum of 10⁻³ -10⁻⁷ Torr, temperature of the cruciblesare independently controlled so that their temperatures are controlledsuch that the luminescent layer will have a stoichiometric composition.Thus, column crystals having a uniform distribution of the activator canbe deposited on a substrate 15 according to the following steps.

Suppose now that substances A and B are formed on a substrate bymultisource vapor deposition process in which independent evaporationsources comprising independent crucibles respectively containingsubstances A and B are used.

Let us denote the vapor pressure of the substance A at a giventemperature TA by PA. Then, when the degree of vacuum PO (pressure) inthe vacuum chamber is PO<PA, by selecting TS in a relation TS>TA suchthat the substrate temperature TA will satisfy a relation PAS>PA wherePAS represents the vapor pressure of substance A at a substratetemperature TS so that even if the substance A is evaporated, it is notdeposited on the substrate.

On the other hand, when the vapor pressure PABS of a compound of AB at atemperature TS of substances A and B is selected such that PABS<PO,where substance B presents on the substrate (PBS<PO), the substances Aand B react with each other on the substrate whereby crystals of acompound AB grows. At this time, when element B (or A) presenting on thesubstrate combines with arriving element A (or B), the elements aredeposited at a position having lowest potential with the result thatonly the crystalline surfaces grow thereby forming column crystals.

Where the activator is constituted by a compound, more efficientluminescent layer can be formed by evaporating respective elements ofthe compound in independent crucibles and then independently controllingthe quantity of evaporation of respective elements.

For example, respective constituting elements Zn and S of ZnS utilizedat the host material of the luminescent layer, and constituting elementsTb and F of the activator TbF₃ are put in independent crucibles (Zn, S,Tb, TbF₃). When the temperature of the crucibles are independentlycontrolled so as to control evaporation quantities such that theresultant luminescent layer will have a stoichiometric structure, it ispossible to deposit on the substrate column crystals having a uniformdistribution of the activator.

In a thin film EL device formed by sequentially laminating a transparentelectrode, a first insulating layer, a luminescent layer, a secondinsulating layer and a back electrode on a substrate according to thisinvention, a third dielectric layer may be interposed between thetransparent electrode and the first insulating layer, the thirdinsulating layer aaving a refractive index intermediate of the indicesof the transparent electrode and of the first insulating layer.

With this construction, it is possible to decrease the refractive indexat the interface between the first insulating layer and the transparentelectrode without decreasing the dielectric constant of the firstinsulating layer. As a consequence, it is possible to decreasereflection thereby enabling to efficiently deriving out light from theluminescent layer.

For example, by denoting the refractive index of the transparentelectrode by n₀ =2, that of the first insulating layer by n₂ =1 andwhere a third insulating layer having a refractive index n₁ =3 isinterposed between the transparent electrode and the first insulatinglayer, the reflective index R₄ between the transparent electrode and thethird insulating layer is expressed as follows according to present law.##EQU1## The reflection index between the third insulatin layer and thefirst insulating layer is expressed by ##EQU2## Consequently the totalrefractive index becomes 6% so that the transmission efficiency becomes94%.

In contrast, in the absence of the third insulating layer the refractiveindex between the transparent electrode and the first insulating layeris expressed by ##EQU3## and the transmission efficiency becomes 89%.

Comparison of these transmission efficiencies shows that thetransmission efficiency can be greatly improved by the addition of thethird dielectric layer.

The embodiment shown in FIG. 3 has a laminated construction wherein aluminescent layer 21 is made of a thin layer of a thickness of 5000 Å ofcolumn polycrystalline structure made of host material ZnS containing0.4 at. % Mn acting as the activator (hereinafter merely termed ZnS: 0.4at. % Mn).

More particularly, on a transparent glass substrate 22 having athickness of 1 micron are sequentially laminated a transparent electrode23 having a thickness of 0.3 micron and made of tin oxide (SnO₂) or thelike, a first insulating layer 24 having a thickness of 0.5 micron andmade of tantalum oxide (Ta₂ O₅), the luminescent layer 21, a secondinsulating layer 25 having a thickness of 0.5 micron and made of Ta₂ O₅and a back electrode 26 made of an aluminum film having a thickness of0.5 micron.

As shown in FIG. 4, the equivalent circuit of this thin film EL devicecan be shown by three serially connected capacitors respectivelyconstituted by the first insulating layer 24, the luminescent layer 21and the second insulating layer 25.

A method of manufacturing the thin film EL device will now be describedas follows.

As shown in FIG. 5A, a transparent electrode 23 consisting of SnO₂ isformed on a transparent glass substrate by sputtering.

Then as shown in FIG. 5B, a first insulating layer 24 consisting of atantalum oxide, oxide layer is formed on the glass layer 23 bysputtering.

Then a vapor deposition apparatus as shown in FIG. 2 is used and Zn, Sand Mn are put in discrete crucibles and the vacuum in the vacuumchamber is selected to be 10⁻⁵ Torr. Then their temperature of the threecrucibles 12, 13, 14 are independently controlled to evaporate Zn, S andMn such that the luminescent layer would have a stoichiometric structurewhile at the same time the temperature TS of the glass substrate 15 isselected to a suitable value in a range of 100°-1000° C. so as to form aluminescent layer 21 (see FIG. 5c) consisting of ZnS column polycrystalshaving a uniform distribution of the activator Mn.

After that, as shown in FIG. 5d, a second insulating layer 25 consistingof tantalum oxide is formed on the luminescent layer 21 by sputtering.

Finally as shown in FIG. 5e, back electrodes 26 are formed by formnng analuminum film by vacuum vapor deposition technique followed bypatterning with lithophotoetching process.

The lumineseent layer of the thin film EL device has an excellentcrystalline nature as evidenced by the result of X-ray diffraction shownin FIG. 6a. For comparison, the result of X-ray difraction of a ZnS:Mnlayer formed by prior art electron beam vapor deposition method is shownin FIG. 6b. Comparison of the characteristics shown in FIGS. 6a and 6bshows that the luminescent layer formed by the method of this inventionhas better crystalline property.

The thin film EL device of this invention is driven by applying an ACelectric field across the transparent electrode and the back electrode,and its voltage brightness characteristic a is shown in FIG. 7 togetherwith that b of a prior art thin film EL device. Comparison of thesecharacteristics shows that the thin film EL device of this invention toproduce the same brightness with a voltage of about 1/2 of that requiredfor the prior art thin film EL device. In other words, the element ofthis invention can produce a high brightness with low voltage.

Where an insulating film whose dielectric constant is much greater thanthat of the luminescent layer 21, for example, barium titanate BaTiO₃,is used for the first insulating layer 24 and the second insulatinglayer 25 the voltage--brightness characteristic is shown by a curve c inFIG. 7 showing that the element can be driven with a voltage of 1/3 or1/4 of the voltage necessary for the prior art thin film EL device, asshown by curve b in FIG. 7. Denoting the dielectric constants of thefirst and second dielectric layers 24 and 25 and at the luminescentlayer 21 by εr1, εr2 and εl respectively, the relation among thesedielectric constants becomes εr1 εr2>>ε1 (see FIG. 4). Consequently, therelation among their capacitances can be expressed by Cr1, Cr2 and C1becomes Cr1, Cr2>>C1 so that almost all voltage applied to the devicefrom outside would be applied across the luminescent layer.

As above described with the thin EL element of this invention a lowvoltage of less than about 100 V is sufficient to obtain a brightness ofaoout 20 ft-l (foot Lambert) which is valuable for practical use.

Although in the foregoing embodiment a ZnS:Mn film was used as theluminescent layer, it should be understood that the invention is notlimited to such specific materials. For example, such other columnerpolycrystals can be used as ZnS: 0.1-1 at. % TbF₃, ZnS: 0.1-1 at. % SmF₃column polycrystals wherein ZnS is used as the host material and onlythe activator is substituted by terbium fluoride (TbF₃) or samariumfluoride (SmF₃) and such other column polycrystals as calcium sulfide(CaS): 0.1-1 at. % europium (Eu), strontium sulfide (SrS): 0.1-1 at. %cerium (CeF₃) can also be used.

In the foregoing embodiment, for the purpose of forming a columnpolycrystalline film of ZnS:Mn three crucibles respectively contrivingZn, S and Mn were used as the sources of vapor deposition, anycombination of ZnS, S, Mn; Zn, S, MnS and Zn, S, ZnS, Mn can be used.

Where such compound as TbF₃ is used as the activator, the constitutingelements of the host material and the constituting elements of anactivator and compound of the constituting elements Zn, S, Tb, TbF₃ canbe put in independent crucibles so as to evaporate Zn, S, Tb and TbF₃ byindependently controlling the temperatures of the crucibles such thathost material and activator of the luminescent layer will have thestoichiometric composition and that the impurity concentration wouddhave a predetermined value. At the same time, the substrate temperatureis selected to a suitable value in a range of 100°-1000° C. (for thecrucibles containing Tb and TbF₃, temperatures of 100°-110° C. arepreferred). Then a luminescent layer consisting of ZnS columnpolycrystals haiing a uniform distribution of the activator TbF₃ can beformed. Preferably, the crucibles have a shutter that can be opened andclosed.

With such method, it is possible to obtain a luminescent layer havingconstant and stable concentration distribution.

Furthermore, the thin film EL device can be driven by applying analternating current electric field across the transparent electrode andthe back electrode but it can produce the same brightness under avoltage of 1/2 of that of the prior art film EL. In other words, ahigher brightness can be contained with lower voltage. Thus a green thinfilm EL device can be firstly obtained according to this invention.

By interposing a yttrium (Y₂ O₃) layer acting as a third insulatinglayer, having an intermediate refractive index between the firstinsulating layer 24 and the transparent electrode 23, the difference inthe refractive index at the interface between the first insulating layerand the transparent electrode can be reduced and the reflection can alsobe reduced. Thereby enabling to efficient deriving out the light emittedby the luminescent layer. Accordingly it is possible to further decreasethe driving voltage.

The activator is not limited to TbF₃, and such other compounds as SmF₃,Sn₂ S₃ and Tb₂ S₃ can also be used. Further, other substances then ZnScan be used as the host material.

Although in the foregoing embodiment, for the purpose of preparingcolumn polycrystalline film of ZnS:T₆ F₃, four crucibles respectivelycontaining ZnS, Tb, and TbF₃ were used as the vapor deposition sources,four crucibles respectively containing Zn, TbS, TbF₃ and S or Zn, Tb,ZnF₂ and S can also be used.

Furthermore the thin film EL device of this invention can be used as alight source for display, illumination, writing, reading and erasing ofsignals into and out of light recording medium.

In another embodiment, shown in FIG. 8, the thin film EL devicecomprises a lamination of a transparent glass substances 31 having athickness of 1 micron, a transparent electrode 32 made of tin oxide(SnO₂) having a thickness of 3 microns, a third insulating layer 33 madeof yttrium oxide (Y₂ O₃) having a thickness of 1.0 micron, a firstinsulating layer made of tantal pentaoxide Ta₂ O₅, having a thickness of0.1 micron, a luminescent layer 35 made of zinc sulfide (ZnS):manganese(Mn) having a thickness of 0.5 micron, a second insulating layer 36 madeof tantal pentaoxide having a thickness of 0.5 micron and a backelectrode made of an aluminum film having a thickness of 0.5 micron.

This thin film EL device is driven by an AC electric field appliedacross the transparent electrode and the back electrode and itsvoltage--brightness characteristics a is shown in FIG. 9 together withthat b of a prior art thin film EL device not provided with the thirdinsulating layer. Comparison of these characteristics clearly shows thatthin film EL device of this invention has large brightness than theprior art thin film EL device. In FIG. 9, the ordinate shows brightnessand the abscissa the applied voltage in volt.

As above described, by interposing a third insulating layer, thedifference in the refractive index at the interpose between the firstinsulating layer and the transparent electrode can be decreased withoutdecreasing the dielectric constant of the first insulating layer wherebylight reflection is decreased for efficiently driving out the lightemitted by the luminescent layer.

In the above described embodiment, the third insulating layer had asingle layer construction, it may be constructed by multiple layerswhose refractive indices are gradually increased from transparentelectrode side to the second electrode side for gradually varying torefractive index.

It is also clear that materials for preparing various layers and filmscan be changed accordingly.

What is claimed is:
 1. A thin film EL device having increased brightnesscomprising a transparent electrode, a first insulating layer, aluminescent layer, a second insulating layer and a back electrode, saidluminescent layer being constituted by column polycrystals.
 2. The thinfilm EL device according to claim 1 wherein said luminescent layer ismade of material consisting of a II-VI compound acting as a hostmaterial and an activator added to said host material.
 3. The thin filmEL device according to claim 2 wherein said activator is constituted ofa single element.
 4. The thin film EL device according to claim 2wherein said activator is constituted of a compound.
 5. The thin film ELdevice according to claim 2 wherein said luminescent host material isZnS.
 6. The thin film EL device according to claim 3 wherein saidactivator is Mn.
 7. The thin film EL device according to claim 4 whereinsaid activator is selected from the group consisting of terbium fluoride(TbF₃), samarium fluoride (SmF₃) and cerium fluoride (CeF₃).
 8. The thinfilm EL device according to claim 1 further comprising anotherinsulating layer interposed between said transparent electrode and saidinsulating layer, said another insulating layer having a refractiveindex intermediate of those of said transparent electrode and saidinsulating layer.
 9. A thin film EL device having increased brightnesscomprising a transparent electrode, a first insulating layer, aluminescent layer and a back electrode, wherein a second insulatinglayer is interposed between said transparent electrode and saidinsulating layer, said second insulating layer having a refractive indexintermediate of said transparent electrode and of said insulating layer.10. A thin film EL device having increased brightness, comprising:atransparent electrode; a first insulating layer; a luminescent layer;back electrode, and a second insulating layer interposed between saidtransparent electrode and said first insulating layer, said secondinsulating layer being constituted by a plurality of laminated layers,refractive indices thereof varying monotonously.
 11. A thin film ELdevice according to claim 10, wherein said luminescent layer isconstituted by column polycrystals.